Because he actually said it, according to this: https://en.wikiquote.org/wiki/John_von_Neumann#:~:text=Young%20man%2C%20in%20mathematics%20you%20don%27t%20unde

Because he actually said it, according to this: https://en.wikiquote.org/wiki/John_von_Neumann#:~:text=Young%20man%2C%20in%20mathematics%20you%20don%27t%20understand%20things.%20You%20just%20get%20used%20to%20them https://en.wikiquote.org/wiki/John_von_Neumann#:~:text=Young....